U.S. patent number 4,298,324 [Application Number 06/077,783] was granted by the patent office on 1981-11-03 for apparatus for molding particulate expandable thermoplastic resin material using microwave heating.
This patent grant is currently assigned to Isobox-Barbier. Invention is credited to Joel Soulier.
United States Patent |
4,298,324 |
Soulier |
November 3, 1981 |
Apparatus for molding particulate expandable thermoplastic resin
material using microwave heating
Abstract
Molding of expanded plastic material such as dielectric material
by microwave radiation is effected in a mold in which the part of
the mold which contacts the material to be molded is of a material
having high dielectric losses and capable of absorbing microwave
radiation. Such mold portion preferably comprises a resin
containing a small amount of carbon black.
Inventors: |
Soulier; Joel (Ivry La
Bataille, FR) |
Assignee: |
Isobox-Barbier (Bannalec,
FR)
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Family
ID: |
9195202 |
Appl.
No.: |
06/077,783 |
Filed: |
September 21, 1979 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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842314 |
Oct 14, 1977 |
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Foreign Application Priority Data
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Sep 9, 1977 [FR] |
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77 27302 |
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Current U.S.
Class: |
425/174.8E;
249/79; 264/51; 425/DIG.13; 249/81; 264/337; 425/4R; 264/402;
264/417 |
Current CPC
Class: |
B29C
33/04 (20130101); B29C 33/06 (20130101); B29C
33/40 (20130101); B29C 33/56 (20130101); B29C
44/3415 (20130101); B29C 2035/0855 (20130101); B29K
2707/04 (20130101); B29K 2709/08 (20130101); B29K
2863/00 (20130101); Y10S 425/013 (20130101); B29K
2105/04 (20130101) |
Current International
Class: |
B29C
33/04 (20060101); B29C 33/56 (20060101); B29C
33/40 (20060101); B29C 33/06 (20060101); B29C
35/08 (20060101); B29D 027/00 () |
Field of
Search: |
;425/174,174.8R,174.8E,4R,DIG.13 ;264/26,45.6,25,51,337
;249/79,81 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2149529 |
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Mar 1973 |
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FR |
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1098312 |
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Jan 1968 |
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GB |
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1403392 |
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Aug 1975 |
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GB |
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Other References
Lanigan, W. J., "Microwave Curing of Flexible Polyurethane Foam
Mouldings", in British Plastics, Oct. 1963, pp. 562-565. .
"Microwave Power For Fast Curing", Section: Foam Moulding, in
Rubber & Plastics Age, vol. 44, No. 5, May 1963, p.
525..
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Primary Examiner: Anderson; Philip
Attorney, Agent or Firm: Oblon, Fisher, Spivak, McClelland
& Maier
Parent Case Text
This is a continuation, of application Ser. No. 842,314, filed Oct.
14, 1977 now abandoned.
Claims
What is claimed as new and desired to be secured by Letters Patent
of the United States is:
1. A device for molding particulate expansible plastic dielectric
material by microwave radiation, which comprises:
a mold body, in combination with a press and a resonant cavity
having wall members wherein the mold body includes a first portion
comprising wall surfaces which define the mold cavity and which
contact said plastic material to be molded said portion being
formed of a resin containing microwave radiation-conducting carbon
black which has high dielectric losses and which is
microwave-absorbent and wherein the remaining portion of the mold
body is made of a material which is essentially transparent to or
transmissive of microwave radiation; and
means for applying microwave radiation to said mold body.
2. The device of claim 1 wherein said resin contains microwave
radiation-conducting carbon black in a proportion of from about 1
to 5% by weight of said resin.
3. The device of claim 1 wherein said remainder of the mold body
material comprises a resin with a filler or fibres of pure silica,
having a low loss angle.
4. The device of claim 1 wherein said remainder of the mold body
material comprises a resin with a glass fibre filler.
5. The device of claim 1 wherein said remainder of the mold body
comprises a resin with a filler of pure silica and glass
fibres.
6. The device of claim 1 wherein said fluid circulation means is
located in the mold body.
7. A mold assembly for molding particulate expansible plastic
dielectric material by microwave radiation which comprises:
a mold body, in combination with a press and a resonant cavity
having wall members wherein the mold body includes a first portion
comprising wall surfaces which define the mold cavity and which
contact the material to be molded said portion being formed of a
resin containing microwave radiation-conducting carbon black which
has high dielectric losses and which is microwave-absorbent and
wherein the remaining portion of mold body is made of a material
which is essentially transparent to or transmissive of microwave
radiation;
means for applying microwave radiation to said mold body; and
means disposed within and connected to said resonant cavity wall
members for circulation of heat-exchange fluid, in operative
association with the mold body.
8. The mold assembly of claim 7 wherein said microwave-absorbent
carbon black is from about 1 to 5% by weight of said resin.
9. An apparatus for molding particulate expansible plastic
dielectric material by microwave radiation, the apparatus
comprising:
a press;
a resonant cavity, having wall members, formed within the
press,
at least one mold body within the resonant cavity, wherein the mold
body includes a first portion comprising wall surfaces which define
the mold cavity and which contact said plastic material to be
molded said portion being formed of resin containing microwave
radiation-conducting carbon black which has a high dielectric loss
and which is microwave-absorbent and wherein the remaining portion
of the mold body is made of material which is essentially
transparent to or transmissive of microwave radiation; and
means for applying microwave radiation to said mold body.
10. The apparatus of claim 9 wherein said resin contains microwave
radiation conducting carbon black in a proportion of from about 1
to 5% by weight of said resin.
11. The apparatus of claim 9 wherein said remainder of the mold
body material comprises a resin containing a filler of pure silica
fibres.
12. The apparatus of claim 9 wherein said remainder of the mold
body material comprises a resin with a glass fibre filler.
13. The apparatus of claim 9 which further comprises an
electromagnetic field agitating means disposed in the resonant
cavity operatively adjacent to the outlet of the waveguide
thereinto, thereby to prevent the formation of standing waves
therein.
14. The apparatus of claim 13 further comprising deflector means
wherein said agitating means is cooperable with said deflector
means for deflecting the microwave radiation into the resonant
cavity adjacent said agitating means.
15. The mold assembly of claim 7, wherein said remainder of the
mold body material comprises a resin with a filler of fibres of
pure silica, having a low loss angle.
16. The mold assembly of claim 7, wherein said remainder of the
mold body material comprises a resin with a glass fibre filler.
17. The mold assembly of claim 7, wherein said remainder of the
mold body material comprises a resin with a filler of pure silica
and glass fibres.
18. The apparatus of claim 9, wherein said remainder of the mold
body material comprises a resin with a filler of pure silica and
glass fibres.
Description
BACKGROUND OF THE INVENTION
In recent times there has been considerable development and
increase in the use of plastic materials for example in the
building industry, the refrigerating industry, in the production of
unsinkable members e.g. for boats, in packaging and other product
conditioning, and the aeronautical and automobile industry. One
consequence of this increased use has been a certain amount of
progress in improving apparatus and processes for producing
expanded plastic material. For example, one step along this route
was the use of a high-frequency field to melt the synthetic resin,
as disclosed for example in French Pat. Nos. 2 149 529, 1 217 351,
2 045 888 and 2 186 344, and U.S. Pat. No. 3,377,653.
A further improvement lay in the use of ultra-high frequency or
microwave radiation (generally 500 to 300,000 MHz) which made it
possible to produce plastics foams in an economical manner. This
process does not require a capacative circuit and does not cause
voltage cracking, the ultra-high frequency (referred to herein as
U.H.F.) field energizing a resonant cavity or a resonant waveguide.
A process of this nature is disclosed in French Patent applications
Nos. 76 01049 and 76 31899 in the name of the assignees of the
present application, which set forth an operating procedure of
simultaneously introducing beads or pearls of plastic materials
which had first been pre-expanded, and water, into a mold disposed
in a resonant cavity. This mode of operation produces conditions
which permit the formation of vapour in situ, and excellent welding
of the beads or pearls to each other, with a good degree of
distribution of the heat involved in melting the material.
However, this process suffers from the disadvantage that the molds
used are of a material which is transparent to or transmissive of
the U.H.F. radiation or wich has a low absorption capacity in
respect of such radiation. A consequence of this is a certain lack
of uniformity in the heating action and an increase in the amount
of power which has to be employed due to the fact that it is
necessary to heat the walls of the mold in each molding cycle to
achieve heating uniformity. This process therefore not only suffers
from poor power efficiency but also tends to suffer from an
extended operating cycle due to the need for additional heating of
the mold.
SUMMARY OF THE INVENTION
An object of the invention is to provide a mold which better
fulfills the practical requirements of microwave molding
operation.
A further object of the invention is to provide a mold for molding
expanded plastics materials by microwave radiation, which provides
an excellent temperature gradient and heat distribution, and which
can achieve a substantial saving in the amount of power required
for melting the surface of the plastic material in the mold.
According to the present invention, these and other objects are
achieved by a mold and a molding apparatus for molding expanded
plastic materials, such as dielectric materials, by microwave
radiation wherein the part of the mold cavity which contacts the
plastic material to be molded comprises a material having high
dielectric losses, and capable of absorbing microwave radiation.
The material is preferably a resin containing a small amount of
microwave radiation-conducting carbon black. The amount of carbon
black in the resin is preferably from about 1 to 5%.
The body of the mold may be made of a material which does not
absorb microwave radiation, preferably a resin with a filler or
reinforcement of fibres of pure silica and/or glass with a low loss
angle.
A circuit for the circulation of a heat-exchange fluid is
associated with the body of the mold, such fluid preferably having
low dielectric losses and a high heat capacity per unit of weight.
The heat-exchange fluid circulation circuit is preferably disposed
in the body of the mold itself, but the fluid circulation circuit
may additionally or alternatively be in the form of a jacket or
sleeve assembly on the mold body.
The mold may be cooled by forced air circulation around its outside
walls, and such circulation may be achieved by means of blades or
vanes on the outside walls of the mold.
Furthermore, the mechanical parts of the mold, such as injectors,
vents, or risers, or air-gates and the like can be made e.g.
machined of polytetrafluoroethylene (known under the Trade Name of
TEFLON) and/or vitroceramics.
In the molding apparatus of the invention, the mold is disposed in
a resonant cavity mounted in a press of which one half is fixed and
the other half is movable. Such installations are described in the
abovequoted French patent applications Nos. 76 01049 and 76 31899.
In a preferred form of the apparatus, a portion of the fixed part
of the press is excited by a waveguide which is fed by an microwave
generator, while the movable part of the press carries the mold
assembly. It will be apparent however that the reverse arrangement
may be employed without disadvantage.
In the apparatus according to the invention, a minimum distance
must be maintained between the mold and the wall of the resonant
cavity, and this distance is preferably not less than 65 mm. An
electromagnetic field agitating means may be disposed at the outlet
of the waveguide into the resonant cavity, the effect of which
agitating means is to prevent the formation of standing waves.
A process for molding articles of plastic material in molds in
accordance with the invention provides that beads of plastic
materials, which may possibly contain a swelling agent or a
volatile liquid blowing agent and which may possibly have been
pre-expanded before being used in the mold, are introduced into the
mold according to the invention, possibly at the same time as a
polar liquid such as water. The mold may be a two-part mold, in a
suitable form of press. The beads of plastic material in the mold
are subjected to the action of microwave radiation for a period of
from 5 to 1,800 seconds, in order to produce molded articles of
expanded plastic materials having walls of any desired thicknesses,
in particular thin-walled articles.
The invention is concerned only with molds for producing expanded
plastic material articles as set out above, but other
installations, processes and production lines for producing molded
articles of expansible plastic material, which include the
principles of the present invention, together with the molded
articles produced by such molds and installations according to the
invention are not excluded.
BRIEF DESCRIPTION OF THE DRAWINGS
Various other objects, features and attendant advantages of the
present invention will be more fully appreciated as the same
becomes better understood from the following detailed description
when considered in connection with the accompaying drawings in
which like reference characters designate like or corresponding
parts throughout the several views, and wherein:
FIG. 1 is a detail cross-sectional view of part of a mold according
to the invention,
FIG. 2 is a general diagrammatic view of apparatus for molding
articles of expanded plastic materials, including a plurality of
molds according to the invention, and
FIG. 3 is a diagrammatic cross-sectional view of part of apparatus
for producing thin-walled molded articles such as cups.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Reference will first be made to FIG. 1 which shows a sectional view
of part of a mold 4 for molding an expanded plastic material
article. The mold 4 has a mold cavity defined at least in part by a
wall portion 1 comprising a material which has high dielectric
losses and which is capable of absorbing microwave radiation. The
wall portion 1 is preferably formed by a surface coating or layer
of epoxy resin containing microwave radiation-conducting carbon
black (for example "Vulcan XC 72 R" by CABOT). The resin contains a
small proportion of carbon black relative to the amount of resin,
for example preferably from about 1 to 5% by weight. The wall
portion 1 of microwave radiation-absorbing material will generally
be provided where the material to be molded comes into contact with
the mold, i.e. generally on the whole inside surface defining the
mold cavity.
The thickness of the mold wall portion 1 is determined by the
desired amount of heat to be applied to the plastic material in the
mold, and thus the thickness of the wall of the molded article. It
is generally advantageous for the thickness of the coating which
forms the mold cavity wall portion 1 to be from about 0.5 to 2 mm,
although it may vary according to requirements.
The body 3 of the mold 4 is advantageously made of an epoxy resin
with a filler of fibres of pure silica and/or glass with a low loss
angle (tg.sup..delta. .ltoreq.1.10.sup.-4 for a frequency f=2.45
GHz), which does not absorb microwave radiation. The thickness of
the silica and/or glass fibre-reinforced resin body 3 of the mold
is advantageously from 3 to 5 mm, taking into account the molding
pressures which will generally be encountered in operation.
The mold 4 also has means for the circulation of a heat-exchange
fluid, including for example a silicone oil as produced by
Rhone-Poulenc and known as 47 V 10 RHODORSIL. The fluid-circulation
means of the FIG. 1 mold comprises a passage arrangement 2 within
the mold body 3, to achieve efficiency in the heat-exchange action,
by being adjacent to the wall portion 1 as shown. It will be
appreciated however that a passage arrangement for such a
heat-exchange fluid circulation may additionally or alternatively
be provided in an external jacket or sleeve assembly on the mold
body 3, depending inter alia on the nature of the molding operation
and the article to be molded.
The heat-exchange fluid preferably has low dielectric loss, and a
high heat capacity relative to its weight.
External cooling of the mold may advantageously be effected by
means of a forced air draught, for example by mounting blades or
vanes on the outside surface of the mold.
Reference will now be made to FIG. 2 which shows a diagrammatic
view of an apparatus for molding expanded plastic materials, in
particular dielectric materials. This apparatus includes a mold
assembly providing a plurality of molds 4 as shown in FIG. 1, and a
press having a movable portion of plate 6 and a fixed portion or
plate 7. The movable portion 6 may be moved for example by a
pneumatic jack, or any other suitable means.
The mold assembly 4 is disposed in a resonant cavity 11 mounted in
the press between the two portions 6 and 7 of which the movable
portion 6 carries part of the cavity 11 and one part of the
two-part mold assembly, providing the mold cavities. The part of
the cavity 11 on the fixed portion 7 of the press is connected to a
microwave waveguide 9 which is fed by a microwave generator (not
shown), to energize or excite the portion 7.
The two parts of the mold assembly 4 disposed in the resonant
cavity 11, as shown clearly in FIG. 2, are supported on respective
support assembly 12, and it will be understood that the support
assemblies 12 are also effective to provide an adjustment in
respect of the position of the molds 4, relative to the movable
part 6 of the press, to ensure proper closure thereof.
Mounted within the resonant cavity 11 adjacent to the outlet end
opening of the microwave waveguide 9 is a means 10, shown as a
rotary blade assembly driven by a motor, for agitating the
microwave field, to thereby prevent the formation of stationary or
standing waves. Disposed adjacent to the field agitating means 10
is a deflector 8 illustrated as a plate inclined downwardly towards
the means 10 which is disposed below the waveguide 9 outlet, to
direct the waves towards the means 10 thereby to enhance the
efficiency of the agitation action.
The apparatus of FIG. 2 also has an injector 5 which is in
communication with the mold cavities of the molds 4, for injecting
into the mold cavities plastic material in bead or pearl form,
which may have been subjected to a pre-expansion operation before
being injected and which may possibly contain the swelling agent or
volatile liquid blowing agent. The injector 5 is preferably made of
a material having low dielectric losses.
The molds 4 and the wall of the resonant cavity 11 must be spaced
from each other by a given minimum distance, and it has been found
that this minimum distance is advantageously about 65 mm.
Still referring to FIG. 2, reference numeral 13 denotes flow means
for introducing a heat-exchange fluid which is to circulate in the
internal passage arrangement 2 of the molds 4 and/or in the
external jacket or sleeve assembly, where employed, as mentioned in
relation to FIG. 1. The flow means 13 are preferably also made of a
material with low dielectric losses. Reference numeral 14 denotes
flow means for a flow of e.g. compressed air, which is provided for
ejecting the molded articles, the means 14 also preferably
comprising a material with low dielectric losses. Additionally, the
flow means 14 may also be used to direct a stream of compressed air
over the mold for cooling by forced air circulation.
It should be noted here that the apparatus of FIG. 2 may be used
for molding any expansible plastic material, non-limiting examples
of which include expansible polystyrene, polyethylene and
polyurethane.
Reference will now be made to FIG. 3 which shows a non-limiting
example of use of the mold according to the invention, for
producing articles with thin walls, illustrated in the form of
beakers or cups. Beads or pearls of expansible plastic materials
are introduced by an injector assembly 15 into the mold cavities.
Each mold is in two parts, a female part 16 and a male part 17, and
the inside surfaces which define the mold cavities are provided
with surface coatings to form the wall portions 1. The surface
coatings 1 are of a material with high dielectric loss and capable
of absorbing microwave reduction, as described above with reference
to FIG. 1. As in the FIG. 2 apparatus, the mold assembly of FIG. 3
is disposed in a press within a resonant cavity to which microwave
radiation is fed.
In the arrangement shown in FIG. 3, the high-loss,
radiation-absorbent wall portions 1 of the molds are about 0.8 mm
in thickness. When the mold cavities are also to be supplied with
polar liquid, for example water, at the same time as the plastic
material is to be introduced into the mold cavities, this liquid
injection operation may be effected by movable injectors (not
shown). It will be noted that the operation of molding plastic
materials according to the present invention can be effected
without introducing a polar liquid, when molding thin-walled
articles. The mold is cooled by convection of forced air draught,
which can advantageously also be introduced by movable injectors
(also not shown in FIG. 3). Blades or vanes 18 may be mounted on
the outside surface of the mold to aid cooling.
By way of example only, with an apparatus comprising eight mold
assemblies mounted in a carrousel arrangement and each comprising
six mold cavities, operated in a molding cycle of a duration of 45
seconds, it is possible to produce 3, 840 cups per hour, each cup
weighing about 2 grams, for 7.6 kg of input material for molding.
The total microwave power consumption of the press plus the
pre-expansion arrangement is about 2.5 kw.
In operation of the above-described apparatus, bead or pearls of
plastic materials are introduced into molds 4 (FIGS. 1 and 2) or
16, 17 (FIG. 3). The plastic materials may contain a swelling agent
and it may have been subjected to a pre-expansion action before
being introduced into the mold cavity. The plastic material may be
introduced simultaneously with a polar liquid, for example water,
to cause the formation of vapour in situ, which is required for
welding the beads of plastic material together. When microwave
radiation is introduced into the resonant cavity 11, the wall part
1 of the mold which is in contact with the beads of plastic
material, such part 1 being made as mentoned above of a material
having relatively high dielectric losses, absorbs a part of the
microwave energy and applies it in the form of heat by heat
conduction to the plastic material contained in the mold cavity or
cavities. In this way the apparatus provides for good heat
distribution.
The material in the mold may be subjected to microwave radiation
for a period of e.g. from 5 to 1,800 seconds, to produce the molded
article.
Accordingly, as the mold cavity wall is capable of absorbing
microwave radiation, it can avoid the formation of vapour
condensate on the walls. This fact, together with the fact that the
mold cavity wall portions are very thin and absorb microwave
radiation, can shorten the mold cooling cycle, and also provide a
considerable saving in regard to the power used.
Furthermore, molds according to the invention can be of very low
industrial cost price, as they can be made by a simple casting
operation, and an apparatus of the carrousel type can often produce
approximately twice as much for the same cost price (i.e. equipment
and mold), relative to conventional prior art processes which
employ a steam boiler and a machined mold. Mechanical parts of the
mold such as injectors or vent means may be made of
polytetrafluoroethylene and/or vitroceramics.
Yet another advantage of the apparatus is that it can provide for a
considerable reduction in the time required for changing the mold,
by virtue of the simplicity of its construction. The apparatus also
makes it possible to mold thin-walled articles of high quality, by
a process referred to as a "dry wall process", i.e. without using a
polar liquid in the mold.
It will be understood that the invention has been described above
with reference to embodiments which are given only by way of
example, and that various modifications and variations may be made
within the scope and spirit of the present invention.
* * * * *